1. Field of the Invention
The present invention relates to a method and apparatus for measuring the eccentricity of a rotating optical disk.
2. Description of the Related Art
Owing to errors in disk manufacturing process or imperfect clamping operation, most rotating optical disks inherently show eccentric rotation to some extent. The eccentricity impairs focusing and tracking control and may make normal data read-out impossible in the worst case.
FIG. 1 depicts a block diagram of a conventional optical disk drive with eccentricity compensation capability, comprising an optical pickup 20 for reproducing recorded signals from an optical disk 10, an R/F unit 30 for equalizing and shaping the RF signals reproduced from the optical disk 10 by the pickup 20, a sled motor 21 for moving the optical pickup 20 along the full length of the disk radius, a spindle motor 11 for rotating the optical disk 10, a drive unit 50 for driving the sled motor 21 and the spindle motor 11, a servo unit 40 for controlling the optical pickup 20 and the drive unit 50 using the rotation speed of the optical disk 10 and focus and tracking error signals outputted from the optical pickup 20, a digital signal processing unit 60 for retrieving original digital data from the binary data stream outputted by the RF unit 30 and for creating track crossing pulse signal (Cout) based on the tracking error signal, a microcomputer 70 for counting the track crossing pulse signal and for adjusting the rotation speed of the optical disk 10 based on the counting result.
When the optical disk 10 is inserted into a disk tray, which is a part of the optical disk drive mechanism, the optical disk 10 is clamped by a clamping device. Then, the microcomputer 50 commands the servo unit 40 to spin the optical disk 10 at a constant angular velocity. Controlled by the servo unit 40, the drive unit 50 provides electric current to rotate the spindle motor 11 and maintains the rotation velocity of the optical disk 10 constant.
The microcomputer 70 activates focusing control and then measures the eccentricity of the rotating optical disk 10. To this end, the digital signal processing unit 60 creates track crossing pulse signal (Cout) using the tracking error signal received from the optical pickup 20. The track crossing pulse signal (Cout) shows state transitions when the optical spot enters and exits the mirror area of the optical disk 10, as shown in FIG. 2.
The microcomputer 70 counts the track crossing pulse signal (Cout) during a predefined time interval and calculates the disk eccentricity using the counted number and disk rotation speed. Based on the obtained eccentricity, the microcomputer 70 performs predefined compensation processes such as servo gain setting.
In the conventional method mentioned above, however, the counted number of the track crossing pulse signal (Cout) may change depending upon the displacement of the objective lens with respect to the optical axis. That is, the calculated disk eccentricity might be incorrect around xe2x80x98Zxe2x80x99 portion of the tracking error signal of FIG. 2, which means that the misalignment between the center of the objective lens and the optical axis is much increased because of the disk vibration.
It is therefore an object of the present invention to provide a method and apparatus for precisely measuring the eccentricity of a rotating optical disk so that appropriate compensation can be performed according to the measured eccentricity.
The apparatus for measuring the eccentricity of an optical disk in accordance with the present invention comprises drive means for rotating the optical disk, reproducing means for reproducing recorded signals from the optical disk, servo means for performing focusing and tracking servo for the optical disk rotated by the drive means, and control means for detecting the amplitude of tracking error signal and for measuring the eccentricity of the optical disk based on the measured amplitude, the measurement being performed while the servo means is activated.
The method for measuring the eccentricity of an optical disk in accordance with the present invention comprises the steps of activating tracking servo while the optical disk is rotating, detecting the center value of tracking error signal and setting the center value as a reference value, and detecting the eccentricity of the optical disk by comparing the center value and the tracking error signal received right after the reference value is set.